HIV establishes a persistent infection despite vigorous host immune responses. However, studies using attenuated viruses in animal models have shown that protection can be conferred and pathogenesis altered, raising hope that appropriately designed immunogens can elicit protection or attenuate the course of infection. This proposal applies insights gained from studies of the SIV transmembrane envelope protein (TM) to test the hypothesis that the TM ecto- and cytoplasmic domains can be modified rationally to alter pathogenesis and the host immune response. Two Aims are proposed, each with in vitro and in vivo components. Aim # 1 - Based on observations that the SIV TM cytoplasmic tail contains endocytosis signals that reduce Env expression on infected cells, these signals will be analyzed and their impact on pathogenesis and the host immune response determined. Preliminary results indicate that viruses with mutations in these signals are profoundly attenuated in nonhuman primates. In vitro, the structure and function of SIV endocytosis signals will be defined and interactions with cellular and viral proteins characterized. In vivo, the effects of mutations in these signals on pathogenesis will be determined in SIV and SHIV models. A primary goal is to determine if viruses lacking endocytosis signals elicit a different immune response, and if this response can be protective. Aim # 2 - Determinants in the TM ectodomain have been identified that increase the stability of the TM/gp120 interaction and the gp120 content on virions, and in this Aim the effect of this alteration on the host immune response will be determined. In vitro, mutations that increase retention of gp120 will be defined and their mechanism of action determined through genetic and structural approaches. In vivo, inactivated viral particles engineered to express increased levels of gp120 will be used in nonhuman primates to determine if qualitatively different immune responses can be generated, and if this response confers protection or alters the natural history of infection from a pathogenic challenge virus. Collectively these Aims provide novel approaches for structure-based alterations in the SIV TM that will impact on pathogenesis and the host immune response.